1. Field of the Invention
The invention relates to an image forming apparatus having a plurality of fixing means.
2. Description of the Related Art
Hitherto, image forming apparatuses of an electrophotographic type have widely been known as image forming apparatuses such as copying apparatus, printer, and the like. Among the electrophotographic type image forming apparatuses, there is an apparatus which can form not only a monochromatic image but also a full-color image. Since the electrophotographic type image forming apparatuses are used in various fields, a demand to realize high picture quality is rising. Smoothness of a surface of an output image formed on recording sheet (paper) can be mentioned as one of factors which decide the picture quality, particularly, a glossy degree of the full-color image. It is strongly required to improve the smoothness. To satisfy such a request, there has been proposed an image forming method of forming a color image by transferring a color toner image made of a thermoplastic resin onto recording sheet having a transparent resin layer made of the thermoplastic resin and fixing the color toner image (for example, refer to Japanese Patent Application Laid-Open Nos. S64-35452 and H05-216322).
A fixing unit using a belt has been proposed as a construction of a fixing unit suitable for the above image forming method (for example, refer to Japanese Patent Application Laid-Open No. H04-362679). In such a belt fixing unit, recording sheet on which an unfixed toner image has been held is thermally pressed by a fixing belt made of a heat resisting film, and the recording sheet is cooled in the state where it has been closely adhered onto the fixing belt. Thus, the toner image is solidified and fixed onto the recording sheet. The recording sheet on which the toner image has been fixed is peeled off from the fixing belt and delivered to the outside.
In this instance, since the toner image is coagulated along a belt surface shape together with a transparent resin layer by the belt fixing unit in the state where the toner image has been embedded in the transparent resin layer of the recording sheet and the whole surface of the recording sheet becomes a smooth surface, a color image having excellent glossiness can be obtained.
As recording sheet having the transparent resin layer, recording sheet formed with a resin layer which is made of a thermoplastic resin whose glass transition temperature is equal to or lower than 358° K. as a main component and whose thickness is equal to about 10 μm has been proposed (for example, refer to Japanese Patent Application Laid-Open No. 2003-084477).
As an image forming apparatus which can form an image onto the recording sheet having the transparent resin layer, there is an apparatus having a first fixing unit and a second fixing unit constructed by a belt fixing unit. In such an image forming apparatus, a first fixing mode and a second fixing mode are switched in accordance with a type of recording sheet. The first fixing mode is a fixing mode for normal recording sheet. In the case of the first fixing mode, the recording sheet to which a toner image has been transferred is conveyed so that it passes through the first fixing unit and does not pass through the second fixing unit. The second fixing mode is a fixing mode for the recording sheet having the transparent resin layer. In the case of the second fixing mode, first, the toner image on the recording sheet having the transparent resin layer is fixed onto the recording sheet by the first fixing unit. Subsequently, the recording sheet is sent to the second fixing unit. In the second fixing unit, the fixing process is executed so as to obtain the state where the toner image has been embedded in the transparent resin layer on the recording sheet.
Ordinarily, the recording sheet after passing through the fixing unit is in the state where a heat has been accumulated in the sheet. Therefore, if the recording sheet passes through a curved conveying path in the heat accumulation state, a curl of the recording sheet occurs. Before the toner image which has been heated and pressed by the fixing unit solidifies, the toner image is rubbed by conveying rollers, guide ribs, and the like, so that such a phenomenon that its rubbing trace appears as a variation in gloss, or the like occurs. Therefore, to suppress the occurrence of such a curl of the recording sheet, rubbing trace of the toner image, or the like, a cooling unit to promptly cool the recording sheet just after the passage through the first fixing unit is provided. In the first fixing mode, just after the recording sheet passed through the first fixing unit, it is promptly cooled.
In the second fixing mode, in a manner similar to the first fixing mode, in the case of promptly cooling the recording sheet which passed through the first fixing unit (recording sheet having the transparent resin layer), the recording sheet which has once been cooled is heated again by the second fixing unit. In order to allow the recording sheet which has once been cooled to be heated again to a predetermined temperature, it is necessary to use a method whereby a heat capacity of the second fixing unit is increased, a target temperature of temperature adjustment is raised, a fixing nip pressure is raised, or the like.
However, in the second fixing unit, in order to increase the heat capacity of the second fixing unit or raise the target temperature of temperature adjustment, an amount of electric power which is supplied to the second fixing unit increases. In other words, electric power consumption in the second fixing unit increases. In the case of raising the fixing nip pressure, it is necessary to increase a torque of a driving motor of a fixing roller or a pressing roller, so that a driving mechanism in the second fixing unit enlarges in size and a deterioration in separating performance of the recording sheet from the second fixing unit, or the like is caused.